What about "Post-Master"? (that sounds like a mailing term lol)
Post-God (to differentiate)
"Master-Poster"
"Expert Poster"
"5000+ Club Member"
"Poster Extrodinaire"
"Posting Elite"
"Insane Member"
Now, this one fits with the current theme of ranks (Extreme member, Jr. Extreme Member, etc.), and we also say that at RAF, you must hand your sanity over at the door. To get over 5000+ posts would officially classify you as not having your sanity at all. I dunno, just some ideas.